1. Field of the Invention
The present invention relates to an image formation method. More particularly, the present invention relates to a method for forming a color image in an electrographic devices such as an electrophotographic copying machine, an electrophotographic printer, and so forth. Among others, the present invention relates particularly to a color image formation method that will be useful for a color image formation system that disposes a plurality of image formation units each for forming a monochromatic image, particularly those image formation units which use a contact type nonmagnetic one-component developing method, in a conveying direction of sheets, and superposes these monochromatic images with one another to form a color image.
2. Description of the Related Art
Various image formation methods are available for forming a color image on an image recording medium such as a sheet of paper, as is well known in the art. An image formation apparatus based on an electrophotographic system employs a method that repeats a process step of forming a toner image of one of the three primary colors (yellow, magenta and cyan) and black for forming a color image on a photosensitive drum and transferring the toner image to the recording sheet, and a method that aligns serially image formation units for forming monochromatic toner images of the three primary colors and black in a sheet conveying direction, superposes the monochromatic images with one another on the recording sheet, and forms the color image (tandem system).
To form a full color image, in particular, a developing machine using a nonmagnetic one-component developer is advantageous from the aspects of the reduction of the size of the developing machine and its cost, high reliability, and so forth. Since the nonmagnetic one-component developer need not use a carrier in combination, means for mixing and agitating the toner is not necessary, transparency is high, and the thickness of the toner film can be reduced. A known developing machine using the one-component developer comprises a developer support for supporting the one-component developer on the surface thereof and carrying the developer along a predetermined circulation route inclusive of a developing region, storage means for storing the one-component developer, and developer feeding means for supplying the one-component developer stored in the developer storing means to the developer support when it comes into contact with the developer support.
The toner used for the prior art image formation apparatus according to an electrophotographic process is generally prepared by dispersing a dye, a pigment, a colorant such as carbon black, into a binder resin made of a natural or synthetic polymer material, and pulverizing and classifying the resulting dispersion to particles having particle diameters of 1 to 30 μm. To conduct satisfactory printing, the toner has excellent properties such as mechanical properties e.g. particle diameter, shape, degree of aggregation, electric properties such as electric resistance, charge amount, etc, thermal properties such as softening point, melting point, etc, optical properties, safety, storability, and so forth. The toners used for the nonmagnetic one-component developing machine generally contain at least one kind of external additive so as to optimize the property values inclusive of the degree of aggregation and the charge amount and to achieve the formation of a high-quality image. In the case of the non-contact type nonmagnetic one-component developing machine in which development of images is made in accordance with a non-contact or non-impression development method developing the image, for example, the physical attraction force is great between the developing roller as the developer support and the toner and between the toner particles by themselves if the toner having a high degree of aggregation is used. Consequently, the flight range of the toner drops, the image density is low, an intermediate tone cannot be generated and the image has low gradation, as is known to those skilled in the art. Therefore, a nonmagnetic one-component developing machine using the toner, to which an additive (generally called an “external additive”) such as silica is added so as to secure fluidity, is known (see, the developing method and apparatus described in Japanese Examined Patent Publication (Kokoku) No. 63-42787, for example). Besides silica, silicon carbide, titanium oxide, etc, are known as the external additive of this kind. A report has been made to the effect that when a toner stipulating the degree of aggregation to 3 to 30% is used, improvement of image quality and prevention of scattering of the toner particles can be achieved simultaneously (see, Japanese Unexamined Patent Publication (Kokai) No. 6-19297, for example). The fluidity of the toner changes during the use of the toner and invites various problems. For example, the image density gradually lowers and the intermediate tone becomes more difficult to obtain. To solve such problems, an image formation method has been reported that decreases the addition amount of the external additive and thus restricts deterioration of fluidity with the passage of time to not greater than 10 in terms of the change amount of the degree of aggregation (see, to the developing machine described in Japanese Unexamined Patent Publication (Kokai) No. 9-197713, for example).
Further, to stabilize the electrostatic charge amount, a method that prevents filming of the toner to the developing roller (see, the electrostatic charge developing toner described in Japanese Unexamined Patent Publication (Kokai) No. 6-59502, for example) and a method that narrows the particle diameter distribution of the toner so as to restrict the fluctuation of the particle diameter distribution of the toner layer on the toner support with consumption of the toner, are known.
In the contact or impression type nonmagnetic one-component developing method in which the developing roller comes into contact with the photosensitive drum as the image support and develops the electrostatic latent image, the toner transfer force created by the electric field generated between the photosensitive drum and the developing roller is stronger than in the developing method in which the photosensitive drum and the developing roller are out of contact from each other. Therefore, even when the toner has relatively low fluidity, image quality is not deteriorated. However, fluidity of the toner must be secured to a certain extent in order to smoothly conduct various processes such as the smooth transfer of the toner inside the developing machine, charging, the formation of the toner layer, and so forth.
Further, there is the problem resulting from the change of fluidity of the toner during the use. That is, there is a problem of a change in the condition of the toner layer formed by a toner layer thickness-limiting blade. The toner layer thickness-limiting blade pushed, at a predetermined pressure, toward the rotating developing roller forms the toner layer on the developing roller. When fluidity of the toner drops, the aggregation force between the toner particles increases. Because the toner cannot be limited to a thin layer, the toner layer thickness becomes larger in this case. Also, the pressure of the toner layer thickness-limiting blade fails to form a uniform toner layer, so that non-uniformity occurs in the toner layer thickness. These changes of the toner with the passage of time invite image defects such as an increase in the printing density, non-uniformity of the density, a drop in reproducibility of the intermediate tone, etc, in the resulting toner image. The problems resulting from the change of the toner with time are more critical in a color image formation apparatus because an actual color image formation apparatus operates in very many cases under a condition where toner consumption does not occur. Color printers, as color image formation apparatuses used in ordinary offices, are mostly used for character printing by using a black toner, and the color toners (yellow, magenta and cyan) are used for only part of the printing. In such an environment, the color toners are kept under the state where their inter-exchange hardly occurs, or does not occur at all. The toners are replaced with the new toners as the toners inside the developing machine are consumed, and deterioration by the stress due to agitation and friction inside the developing machine can be checked up to a certain level. Under the use environment of the color image formation apparatus described above, however, degradation of the toners becomes a serious problem because replacement of the toners does not occur.
Degradation of the toners occurs not only due to the drop of fluidity described above but also due to the drop in the electrostatic charge amount. The drop in the charge amount causes problems such as photographic fog of the base and the occurrence of a negative after-image.
As described above, the fluctuation resulting from degradation of the toners is a serious problem for a color image formation apparatus even when it is a contact type nonmagnetic one-component developing machine. It is therefore very important to keep the toners under the optimum condition in all use environments.